Multistage nanoparticle delivery system for deep penetration into tumor tissue

Optical imaging of tumor microenvironment

Tumor microenvironment plays important roles in tumor development and metastasis. Features of the tumor microenvironment that are significantly different from normal tissues include acidity, hypoxia, overexpressed proteases and so on. Therefore, these features can serve as not only biomarkers for tumor diagnosis but also theraputic targets for tumor treatment. Imaging modalities such as optical, positron emission tomography (PET) and magnetic resonance imaging (MRI) have been intensively applied to investigate tumor microenvironment. Various imaging probes targeting pH, hypoxia and proteases in tumor microenvironment were thus well developed. In this review, we will focus on recent examples on fluorescent probes for optical imaging of tumor microenvironment. Construction of these fluorescent probes were based on characteristic feature of pH, hypoxia and proteases in tumor microenvironment. Strategies for development of these fluorescent probes and applications of these probes in optical imaging of tumor cells or tissues will be discussed in this review paper.

Superior penetration and retention behavior of 50 nm gold nanoparticles in tumors

Nanoparticles offer potential as drug delivery systems for chemotherapeutics based on certain advantages of molecular drugs. In this study, we report that particle size exerts great influence on the penetration and retention behavior of nanoparticles entering tumors. On comparing gold-coated Au@tiopronin nanoparticles that were prepared with identical coating and surface properties, we found that 50 nanoparticles were more effective in all in vitro, ex vivo, and in vivo assays conducted using MCF-7 breast cells as a model system. Beyond superior penetration in cultured cell monolayers, 50 nm Au@tiopronin nanoparticles also penetrated more deeply into tumor spheroids ex vivo and accumulated more effectively in tumor xenografts in vivo after a single intravenous dose. In contrast, larger gold-coated nanoparticles were primarily localized in the periphery of the tumor spheroid and around blood vessels, hindering deep penetration into tumors. We found multicellular spheroids to offer a simple ex vivo tumor model to simulate tumor tissue for screening the nanoparticle penetration behavior. Taken together, our findings define an optimal smaller size for nanoparticles that maximizes their effective accumulation in tumor tissue.

Compact zwitterion-coated iron oxide nanoparticles for in vitro and in vivo imaging

We have recently developed compact and water-soluble zwitterionic dopamine sulfonate (ZDS) ligand coated superparamagnetic iron oxide nanoparticles (SPIONs) for use in various biomedical applications. The defining characteristics of ZDS-coated SPIONs are small hydrodynamic diameters, low non-specific interactions with fetal bovine serum, the opportunity for specific labeling, and stability with respect to time, pH, and salinity. We report here on the magnetic characterization of ZDS-coated SPIONs and their in vitro and in vivo performance relative to non-specific interactions with HeLa cells and in mice, respectively. ZDS-coated SPIONs retained the superparamagnetism and saturation magnetization (M(s)) of as-synthesized hydrophobic SPIONs, with M(s) = 74 emu g(-1) [Fe]. Moreover, ZDS-coated SPIONs showed only small non-specific uptake into HeLa cancer cells in vitro and low non-specific binding to serum proteins in vivo in mice.

In vitro evaluation of dendrimer-polymer hybrid nanoparticles on their controlled cellular targeting kinetics

Although polymeric nanoparticles (NPs) and dendrimers represent some of the most promising cancer-targeting nanocarriers, each of them has drawbacks such as limited tissue diffusivity/tumor penetration and rapid in vivo elimination, respectively. To address these issues, we have designed a multiscale hybrid NP system (nanohybrid) that combines folate (FA)-targeted poly(amidoamine) dendrimers and poly(ethylene glycol)-b-poly(d,l-lactide) NPs. The nanohybrids (∼100 nm NPs encapsulating ∼5 nm targeted dendrimers) were extensively characterized through a series of in vitro experiments that validate the design rationale of the system, in an aim to simulate their in vivo behaviors. Cellular uptake studies using FA receptor (FR)-overexpressing KB cells (KB FR(+)) revealed that the nanohybrids maintained high FR selectivity resembling the selectivity of free dendrimers, while displaying temporally controlled cellular interactions due to the presence of the polymeric NP shells. The cellular interactions of the nanohybrids were clathrin-dependent (characteristic of polymer NPs) at early incubation time points (4 h), which were partially converted to caveolae-mediated internalization (characteristic of FA-targeted dendrimers) at longer incubation hours (24 h). Simulated penetration assays using multicellular tumor spheroids of KB FR(+) cells also revealed that the targeted dendrimers penetrated deep into the spheroids upon their release from the nanohybrids, whereas the NP shell did not. Additionally, methotrexate-containing systems showed the selective, controlled cytotoxicity kinetics of the nanohybrids. These results all demonstrate that our nanohybrids successfully integrate the unique characteristics of dendrimers (effective targeting and penetration) and polymeric NPs (controlled release and suitable size for long circulation) in a kinetically controlled manner.

Plasmonic photothermal therapy increases the tumor mass penetration of HPMA copolymers

Effective drug delivery to tumors requires both transport through the vasculature and tumor interstitium. Previously, it was shown that gold nanorod (GNR) mediated plasmonic photothermal therapy (PPTT) is capable of increasing the overall accumulation of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers in prostate tumors. In the present study, it is demonstrated that PPTT is also capable of increasing the distribution of these conjugates in tumors. Gadolinium labeled HPMA copolymers were administered to mice bearing prostate tumors immediately before treatment of the right tumor with PPTT. The left tumor served as internal, untreated control. Magnetic resonance imaging (MRI) of both tumors showed that PPTT was capable of improving the tumor mass penetration of HPMA copolymers. Thermal enhancement of delivery, roughly 1.5-fold, to both the tumor center and periphery was observed. Confocal microscopy of fluorescently labeled copolymers corroborates these findings in that PPTT is capable of delivering more HPMA copolymers to the tumor's center and periphery. These results further demonstrate that PPTT is a useful tool to improve the delivery of polymer-drug conjugates.

Systemic delivery of modified mRNA encoding herpes simplex virus 1 thymidine kinase for targeted cancer gene therapy

Failure of clinical trials of nonviral vector-mediated gene therapy arises primarily from either an insufficient transgene expression level or immunostimulation concerns caused by the genetic information carrier (e.g., bacteria-generated, double-stranded DNA (dsDNA)). Neither of these issues could be addressed through engineering-sophisticated gene delivery vehicles. Therefore, we propose a systemic delivery of chemically modified messenger RNA (mRNA) as an alternative to plasmid DNA (pDNA) in cancer gene therapy. Modified mRNA evaded recognition by the innate immune system and was less immunostimulating than dsDNA or regular mRNA. Moreover, the cytoplasmic delivery of mRNA circumvented the nuclear envelope, which resulted in a higher gene expression level. When formulated in the nanoparticle formulation liposome-protamine-RNA (LPR), modified mRNA showed increased nuclease tolerance and was more effectively taken up by tumor cells after systemic administration. The use of LPR resulted in a substantial increase of the gene expression level compared with the equivalent pDNA in the human lung cancer NCI-H460 carcinoma. In a therapeutic model, when modified mRNA encoding herpes simplex virus 1-thymidine kinase (HSV1-tk) was systemically delivered to H460 xenograft-bearing nude mice, it was significantly more effective in suppressing tumor growth than pDNA.

Bacteria-responsive multifunctional nanogel for targeted antibiotic delivery

Bacteria-Responsive Multifunctional Nanogel: We developed a bacteria-responsive multifunctional nanogel for targeted antibiotic delivery, in which bacterial enzymes are utilized to trigger antibiotic release by degrading the polyphosphoester core. The mannosylated nanogel preferentially delivers drugs to macrophages and leads to drug accumulation at bacterial infection sites through macrophage transport. This nanogel provides macrophage targeting and lesion site-activatable drug release properties, which enhances bacterial growth inhibition.

Multifunctional to multistage delivery systems: The evolution of nanoparticles for biomedical applications

Nanomaterials are advancing in several directions with significant progress being achieved with respect to their synthesis, functionalization and biomedical application. In this review, we will describe several classes of prototypical nanocarriers, such as liposomes, silicon particles, and gold nanoshells, in terms of their individual function as well as their synergistic use. Active and passive targeting, photothermal ablation, and drug controlled release constitute some of the crucial functions identified to achieve a medical purpose. Current limitations in targeting, slow clearance, and systemic as well as local toxicity are addressed in reference to the recent studies that attempted to comprehend and solve these issues. The demand for a more sophisticated understanding of the impact of nanomaterialson the body and of their potential immune response underlies this discussion. Combined components are then discussed in the setting of multifunctional nanocarriers, a class of drug delivery systems we envisioned, proposed, and evolved in the last 5 years. In particular, our third generation of nanocarriers, the multistage vectors, usher in the new field of nanomedicine by combining several components onto multifunctional nanocarriers characterized by emerging properties and able to achieve synergistic effects.

Fabrication of biopolymeric complex coacervation core micelles for efficient tea polyphenol delivery via a green process

Nanoencapsulation is a promising method to improve the bioavailability of tea polyphenol (TPP). In this work, we adopted a green process to develop a new kind of complex coacervation core micelles (C3Ms) based on biopolymers for efficient tea polyphenol delivery. First, gelatin-dextran conjugate was synthesized using Maillard reaction. Then the C3Ms were produced by mixing gelatin-dextran conjugate with TPP. Variable factors on the self-assembly of the C3Ms were investigated. Under optimal conditions, the obtained C3Ms are of nanosize (average 86 nm in diameter) with narrow distribution. The formation of the C3Ms is attributed to hydrophobic interaction and hydrogen bonding instead of electrostatic interaction. Transmission electron microscope (TEM) and scanning electron microscope (SEM) results showed that C3Ms have a spherical shape with core-shell structure. ζ-Potential measurement suggested that the core is composed of gelatin with TPP, whereas the shell is composed of dextran segments. The encapsulation efficiency of the C3Ms is pH-independent, but the loading capacity is controllable and as high as 360 wt % (weight/weight of protein). In addition, the C3Ms show sustained release of TPP in vitro. MTT assay revealed that the C3Ms have comparable or even stronger cytotoxicity against MCF-7 cells than free TPP.

The use of a tumor metastasis targeting peptide to deliver doxorubicin-containing liposomes to highly metastatic cancer

Tumor metastasis is responsible for 90% of cancer-associated deaths and highly metastatic cancers are more prone to form metastasis foci and acquire the drug resistance. Here, a nanocarrier system (TMT-LS) has been constructed by modification of stealth liposomes with a metastatic cancer specific peptide, using the unmodified stealth liposomes (LS) as the control. The active targeted nanocarriers presented satisfactory particle size (about 100 nm) and drug release characteristics in vitro. Highly metastatic cancer cells (MDA-MB-435S and MDA-MB-231) and non-metastatic cancer cells (MCF-7) were applied as tumor cell models. The highly metastatic cancer cells were found to endocytose more TMT-LS in a faster way than TS, through a receptor-mediated pathway proved by specific receptor inhibition. Co-localization technique indicated the integrity of nanocarriers in cytoplasm. The significant targeting of TMT-LS to highly metastatic tumors was demonstrated in vivo and ex vivo in an orthotopic model as well as in a double tumor-bearing animal model with both metastatic and non-metastatic tumors in the same mouse. Importantly, the active targeted drug delivery system was found to penetrate deeper into tumor mass and have a longer retention within the malignant tissue. Further, TMT-LS greatly facilitated the efficacy of doxorubicin loaded in terms of inhibiting xenograft tumor growth and inducing cancer cell apoptosis, with only minor side effects. Together, the specific nanocarriers hold great potential in the development of nanomedicine for diagnosis and therapy of metastatic tumor.

Nanomedicines based on recombinant fusion proteins for targeting therapeutic siRNA oligonucleotides

The enormous promise of siRNA technology for rational and targeted therapy can only be realized if the inherent problems in terms of pharmaceutical development are overcome. Besides liposomal and polymeric nanoparticles, fusion proteins hold great potential for cell-type specific delivery of siRNA. Consisting of a protein binder and an oligonucleotide complexing domain, fusion proteins are designed for targeted delivery to a certain tissue or organ and subsequent release of the siRNA after cellular uptake. This article focuses on the possibilities and importance of targeting and complexing domains, including polymers and dendrimers. In vitro and in vivo evaluations are discussed with an in-depth view on pharmacokinetic properties. Remaining challenges concerning specificity on the tissue and molecular levels are highlighted.

Cationic nanoparticles have superior transvascular flux into solid tumors: insights from a mathematical model

Despite their great promise, only a few nanoparticle formulations have been approved for clinical use in oncology. The failure of nano-scale drugs to enhance cancer therapy is in large part due to inefficient delivery. To overcome this outstanding problem, a better understanding of how the physical properties (i.e., size, surface chemistry, and shape) of nanoparticles affect their transvascular transport in tumors is required. In this study, we developed a mathematical model for nanoparticle delivery to solid tumors taking into account electrostatic interactions between the particles and the negatively-charged pores of the vessel wall. The model predictions suggest that electrostatic repulsion has a minor effect on the transvascular transport of nanoparticles. On the contrary, electrostatic attraction, caused even by small cationic charges (surface charge density less than 3 × 10(-3) C/m(2)) can lead to a twofold or more increase in the transvascular flux of nanoparticles into the tumor interstitial space. Importantly, for every nanoparticle size, there is a value of charge density above which a steep increase in transvascular transport is predicted. Our model provides important guidelines for the optimal design of nanoparticle formulation for delivery to solid tumors.

Drug targeting to tumors: principles, pitfalls and (pre-) clinical progress

Many different systems and strategies have been evaluated for drug targeting to tumors over the years. Routinely used systems include liposomes, polymers, micelles, nanoparticles and antibodies, and examples of strategies are passive drug targeting, active drug targeting to cancer cells, active drug targeting to endothelial cells and triggered drug delivery. Significant progress has been made in this area of research both at the preclinical and at the clinical level, and a number of (primarily passively tumor-targeted) nanomedicine formulations have been approved for clinical use. Significant progress has also been made with regard to better understanding the (patho-) physiological principles of drug targeting to tumors. This has led to the identification of several important pitfalls in tumor-targeted drug delivery, including I) overinterpretation of the EPR effect; II) poor tumor and tissue penetration of nanomedicines; III) misunderstanding of the potential usefulness of active drug targeting; IV) irrational formulation design, based on materials which are too complex and not broadly applicable; V) insufficient incorporation of nanomedicine formulations in clinically relevant combination regimens; VI) negligence of the notion that the highest medical need relates to metastasis, and not to solid tumor treatment; VII) insufficient integration of non-invasive imaging techniques and theranostics, which could be used to personalize nanomedicine-based therapeutic interventions; and VIII) lack of (efficacy analyses in) proper animal models, which are physiologically more relevant and more predictive for the clinical situation. These insights strongly suggest that besides making ever more nanomedicine formulations, future efforts should also address some of the conceptual drawbacks of drug targeting to tumors, and that strategies should be developed to overcome these shortcomings.

A Single UV or Near IR Triggering Event Leads to Polymer Degradation into Small Molecules

We report two polymers with UV- and NIR-removable end caps that respond to a single light activated event by complete cleavage of the polymer backbone via a self-immolative mechanism. Two photocleavable protecting groups were used to cap the polymers; o-nitrobenzyl alcohol (ONB) and bromo-coumarin (Bhc). GPC and (1)H NMR confirmed complete degradation of the ONB-containing polymer in response to UV. The polymers were formulated into nanoparticles; fluorescence measurements of encapsulated Nile red confirmed release upon photolysis of the endcaps. Contrary to previous work using a similar backbone structure that degrades upon hydrolysis, here, the disassembly process and burst release of the payload are only activated on demand, illustrating the powerful capacity of light to trigger release from polymeric nanoparticles. Our design allows the signal to be amplified in a domino effect to fully degrade the polymer into small molecules. Thus, polymers and nanoparticles can reach maximal degradation without having to use intense and/or long periods of irradiation.

Synthesis of biocompatible gelatinated thioglycolic acid-capped CdTe quantum dots ("jelly dots")

Semiconductor luminescent Quantum Dots (QDs) constitute a growing area of research for biological imaging and other biomedical applications. One of the main challenges is to provide QDs with a biocompatible and easy to functionalize surface while retaining the core optical properties. Gelatine is an excellent candidate for that purpose as it is a very common natural polymer, highly biocompatible and bearing various functional groups. Here we present a simple, one-pot method for manufacturing gelatinated QDs with chosen optical properties.

Pulsed high intensity focused ultrasound increases penetration and therapeutic efficacy of monoclonal antibodies in murine xenograft tumors

The success of radioimmunotherapy for solid tumors remains elusive due to poor biodistribution and insufficient tumor accumulation, in part, due to the unique tumor microenvironment resulting in heterogeneous tumor antibody distribution. Pulsed high intensity focused ultrasound (pulsed-HIFU) has previously been shown to increase the accumulation of (111)In labeled B3 antibody (recognizes Lewis(y) antigen). The objective of this study was to investigate the tumor penetration and therapeutic efficacy of pulsed-HIFU exposures combined with (90)Y labeled B3 mAb in an A431 solid tumor model. The ability of pulsed-HIFU (1 M Hz, spatial averaged temporal peak intensity=2685 W cm(-2); pulse repetition frequency=1 Hz; duty cycle=5%) to improve the tumor penetration and therapeutic efficacy of (90)Y labeled B3 mAb ((90)Y-B3) was evaluated in Le(y)-positive A431 tumors. Antibody penetration from the tumor surface and blood vessel surface was evaluated with fluorescently labeled B3, epi-fluorescent microscopy, and custom image analysis. Tumor size was monitored to determine treatment efficacy, indicated by survival, following various treatments with pulsed-HIFU and/or (90)Y-B3. The pulsed-HIFU exposures did not affect the vascular parameters including microvascular density, vascular size, and vascular architecture; although 1.6-fold more antibody was delivered to the solid tumors when combined with pulsed-HIFU. The distribution and penetration of the antibodies were significantly improved (p-value<0.05) when combined with pulsed-HIFU, only in the tumor periphery. Pretreatment with pulsed-HIFU significantly improved (p-value<0.05) survival over control treatments.

In vivo fluorescence imaging in the second near-infrared window with long circulating carbon nanotubes capable of ultrahigh tumor uptake

Cancer imaging requires selective high accumulation of contrast agents in the tumor region and correspondingly low uptake in healthy tissues. Here, by making use of a novel synthetic polymer to solubilize single-walled carbon nanotubes (SWNTs), we prepared a well-functionalized SWNT formulation with long blood circulation (half-life of ∼30 h) in vivo to achieve ultrahigh accumulation of ∼30% injected dose (ID)/g in 4T1 murine breast tumors in Balb/c mice. Functionalization dependent blood circulation and tumor uptake were investigated through comparisons with phospholipid-PEG solubilized SWNTs. For the first time, we performed video-rate imaging of tumors based on the intrinsic fluorescence of SWNTs in the second near-infrared (NIR-II, 1.1-1.4 μm) window. We carried out dynamic contrast imaging through principal component analysis (PCA) to immediately pinpoint the tumor within ∼20 s after injection. Imaging over time revealed increasing tumor contrast up to 72 h after injection, allowing for its unambiguous identification. The 3D reconstruction of the SWNTs distribution based on their stable photoluminescence inside the tumor revealed a high degree of colocalization of SWNTs and blood vessels, suggesting enhanced permeability and retention (EPR) effect as the main cause of high passive tumor uptake of the nanotubes.

Enzyme-responsive polymeric assemblies, nanoparticles and hydrogels

Being responsive and adaptive to external stimuli is an intrinsic feature characteristic of all living organisms and soft matter. Specifically, responsive polymers can exhibit reversible or irreversible changes in chemical structures and/or physical properties in response to a specific signal input such as pH, temperature, ionic strength, light irradiation, mechanical force, electric and magnetic fields, and analyte of interest (e.g., ions, bioactive molecules, etc.) or an integration of them. The past decade has evidenced tremendous growth in the fundamental research of responsive polymers, and accordingly, diverse applications in fields ranging from drug or gene nanocarriers, imaging, diagnostics, smart actuators, adaptive coatings, to self-healing materials have been explored and suggested. Among a variety of external stimuli that have been utilized for the design of novel responsive polymers, enzymes have recently emerged to be a promising triggering motif. Enzyme-catalyzed reactions are highly selective and efficient toward specific substrates under mild conditions. They are involved in all biological and metabolic processes, serving as the prime protagonists in the chemistry of living organisms at a molecular level. The integration of enzyme-catalyzed reactions with responsive polymers can further broaden the design flexibility and scope of applications by endowing the latter with enhanced triggering specificity and selectivity. In this tutorial review, we describe recent developments concerning enzyme-responsive polymeric assemblies, nanoparticles, and hydrogels by highlighting this research area with selected literature reports. Three different types of systems, namely, enzyme-triggered self-assembly and aggregation of synthetic polymers, enzyme-driven disintegration and structural reorganization of polymeric assemblies and nanoparticles, and enzyme-triggered sol-to-gel and gel-to-sol transitions, are described. Their promising applications in drug controlled release, biocatalysis, imaging, sensing, and diagnostics are also discussed.

Strategies for ocular siRNA delivery: Potential and limitations of non-viral nanocarriers

Controlling gene expression via small interfering RNA (siRNA) has opened the doors to a plethora of therapeutic possibilities, with many currently in the pipelines of drug development for various ocular diseases. Despite the potential of siRNA technologies, barriers to intracellular delivery significantly limit their clinical efficacy. However, recent progress in the field of drug delivery strongly suggests that targeted manipulation of gene expression via siRNA delivered through nanocarriers can have an enormous impact on improving therapeutic outcomes for ophthalmic applications. Particularly, synthetic nanocarriers have demonstrated their suitability as a customizable multifunctional platform for the targeted intracellular delivery of siRNA and other hydrophilic and hydrophobic drugs in ocular applications. We predict that synthetic nanocarriers will simultaneously increase drug bioavailability, while reducing side effects and the need for repeated intraocular injections. This review will discuss the recent advances in ocular siRNA delivery via non-viral nanocarriers and the potential and limitations of various strategies for the development of a ‘universal’ siRNA delivery system for clinical applications.

Size-dependent localization and penetration of ultrasmall gold nanoparticles in cancer cells, multicellular spheroids, and tumors in vivo

This work demonstrated that ultrasmall gold nanoparticles (AuNPs) smaller than 10 nm display unique advantages over nanoparticles larger than 10 nm in terms of localization to, and penetration of, breast cancer cells, multicellular tumor spheroids, and tumors in mice. Au@tiopronin nanoparticles that have tunable sizes from 2 to 15 nm with identical surface coatings of tiopronin and charge were successfully prepared. For monolayer cells, the smaller the Au@tiopronin NPs, the more AuNPs found in each cell. In addition, the accumulation of Au NPs in the ex vivo tumor model was size-dependent: smaller AuNPs were able to penetrate deeply into tumor spheroids, whereas 15 nm nanoparticles were not. Owing to their ultrasmall nanostructure, 2 and 6 nm nanoparticles showed high levels of accumulation in tumor tissue in mice after a single intravenous injection. Surprisingly, both 2 and 6 nm Au@tiopronin nanoparticles were distributed throughout the cytoplasm and nucleus of cancer cells in vitro and in vivo, whereas 15 nm Au@tiopronin nanoparticles were found only in the cytoplasm, where they formed aggregates. The ex vivo multicellular spheroid proved to be a good model to simulate in vivo tumor tissue and evaluate nanoparticle penetration behavior. This work gives important insights into the design and functionalization of nanoparticles to achieve high levels of accumulation in tumors.

Size-dependent localization and penetration of ultrasmall gold nanoparticles in cancer cells, multicellular spheroids, and tumors in vivo

This work demonstrated that ultrasmall gold nanoparticles (AuNPs) smaller than 10 nm display unique advantages over nanoparticles larger than 10 nm in terms of localization to, and penetration of, breast cancer cells, multicellular tumor spheroids, and tumors in mice. Au@tiopronin nanoparticles that have tunable sizes from 2 to 15 nm with identical surface coatings of tiopronin and charge were successfully prepared. For monolayer cells, the smaller the Au@tiopronin NPs, the more AuNPs found in each cell. In addition, the accumulation of Au NPs in the ex vivo tumor model was size-dependent: smaller AuNPs were able to penetrate deeply into tumor spheroids, whereas 15 nm nanoparticles were not. Owing to their ultrasmall nanostructure, 2 and 6 nm nanoparticles showed high levels of accumulation in tumor tissue in mice after a single intravenous injection. Surprisingly, both 2 and 6 nm Au@tiopronin nanoparticles were distributed throughout the cytoplasm and nucleus of cancer cells in vitro and in vivo, whereas 15 nm Au@tiopronin nanoparticles were found only in the cytoplasm, where they formed aggregates. The ex vivo multicellular spheroid proved to be a good model to simulate in vivo tumor tissue and evaluate nanoparticle penetration behavior. This work gives important insights into the design and functionalization of nanoparticles to achieve high levels of accumulation in tumors.

Delivery of molecular and nanoscale medicine to tumors: transport barriers and strategies

Tumors are similar to organs, with unique physiology giving rise to an unusual set of transport barriers to drug delivery. Cancer therapy is limited by nonuniform drug delivery via blood vessels, inhomogeneous drug transport into tumor interstitium from the vascular compartment, and hindered transport through tumor interstitium to the target cells. Four major abnormal physical and physiological properties contribute to these transport barriers. Accumulated solid stress compresses blood vessels to diminish the drug supply to many tumor regions. Immature vasculature with high viscous and geometric resistances and reduced pressure gradients leads to sluggish and heterogeneous blood flow in tumors to further limit drug supply. Nonfunctional lymphatics coupled with highly permeable blood vessels result in elevated hydrostatic pressure in tumors to abrogate convective drug transport from blood vessels into and throughout most of the tumor tissue. Finally, a dense structure of interstitial matrix and cells serves as a tortuous, viscous, and steric barrier to diffusion of therapeutic agents. In this review, we discuss the origins and implications of these barriers. We then highlight strategies for overcoming these barriers by modulating either drug properties or the tumor microenvironment itself to enhance the delivery and effectiveness of drugs in tumors.

Physical and chemical strategies for therapeutic delivery by using polymeric nanoparticles

A significant challenge that most therapeutic agents face is their inability to be delivered effectively. Nanotechnology offers a solution to allow for safe, high-dose, specific delivery of pharmaceuticals to the target tissue. Nanoparticles composed of biodegradable polymers can be designed and engineered with various layers of complexity to achieve drug targeting that was unimaginable years ago by offering multiple mechanisms to encapsulate and strategically deliver drugs, proteins, nucleic acids, or vaccines while improving their therapeutic index. Targeting of nanoparticles to diseased tissue and cells assumes two strategies: physical and chemical targeting. Physical targeting is a strategy enabled by nanoparticle fabrication techniques. It includes using size, shape, charge, and stiffness among other parameters to influence tissue accumulation, adhesion, and cell uptake. New methods to measure size, shape, and polydispersity will enable this field to grow and more thorough comparisons to be made. Physical targeting can be more economically viable when certain fabrication techniques are used. Chemical targeting can employ molecular recognition units to decorate the surface of particles or molecular units responsive to diseased environments or remote stimuli. In this review, we describe sophisticated nanoparticles designed for tissue-specific chemical targeting that use conjugation chemistry to attach targeting moieties. Furthermore, we describe chemical targeting using stimuli responsive nanoparticles that can respond to changes in pH, heat, and light.

The effect of nanoparticle size, shape, and surface chemistry on biological systems

An understanding of the interactions between nanoparticles and biological systems is of significant interest. Studies aimed at correlating the properties of nanomaterials such as size, shape, chemical functionality, surface charge, and composition with biomolecular signaling, biological kinetics, transportation, and toxicity in both cell culture and animal experiments are under way. These fundamental studies will provide a foundation for engineering the next generation of nanoscale devices. Here, we provide rationales for these studies, review the current progress in studies of the interactions of nanomaterials with biological systems, and provide a perspective on the long-term implications of these findings.

Enhanced antitumor efficacy, biodistribution and penetration of docetaxel-loaded biodegradable nanoparticles

To investigate the antitumor effect, biodistribution and penetration in tumors of docetaxel (DOC)-loaded polyethylene glycol-poly(caprolactone) (mPEG-PCL) nanoparticles on hepatic cancer model, DOC-loaded nanoparticles (DOC-NPs) were prepared with synthesized mPEG-PCL by nano-precipitated method with satisfactory encapsulation efficiency, loading capacity and size distribution. The fabricated nano-drugs were effectively transported into tumoral cells through endocytosis and localized around the nuclei in the cytoplasm. In vitro cytotoxicity test showed that DOC-NPs inhibited the murine hepatic carcinoma cell line H22 in a dose-dependent manner, which was similar to Taxotere, the commercialized formulation of docetaxel. The in vivo biodistribution performed on tumor-bearing mice by NIRF real-time imaging demonstrated that the nanoparticles achieved higher concentration and longer retention in tumors than in non-targeted organs after intravenous injection. The immunohistochemical analysis demonstrated that the nanoparticles located not only near the tumoral vasculatures, but also inside the tumoral interior. Therefore, DOC-NPs could penetrate into tumor parenchyma, leading to high intratumoral concentration of DOC. More importantly, the in vivo anti-tumor evaluation showed that DOC-NPs significantly inhibited tumor growth by tumor volume measurement and positron emission tomography and computed tomography (PET/CT) imaging observation. Taken together, the reported drug delivery system here could shed light on the future targeted therapy against hepatic carcinoma.

Targeted polymeric therapeutic nanoparticles: design, development and clinical translation

Polymeric materials have been used in a range of pharmaceutical and biotechnology products for more than 40 years. These materials have evolved from their earlier use as biodegradable products such as resorbable sutures, orthopaedic implants, macroscale and microscale drug delivery systems such as microparticles and wafers used as controlled drug release depots, to multifunctional nanoparticles (NPs) capable of targeting, and controlled release of therapeutic and diagnostic agents. These newer generations of targeted and controlled release polymeric NPs are now engineered to navigate the complex in vivo environment, and incorporate functionalities for achieving target specificity, control of drug concentration and exposure kinetics at the tissue, cell, and subcellular levels. Indeed this optimization of drug pharmacology as aided by careful design of multifunctional NPs can lead to improved drug safety and efficacy, and may be complimentary to drug enhancements that are traditionally achieved by medicinal chemistry. In this regard, polymeric NPs have the potential to result in a highly differentiated new class of therapeutics, distinct from the original active drugs used in their composition, and distinct from first generation NPs that largely facilitated drug formulation. A greater flexibility in the design of drug molecules themselves may also be facilitated following their incorporation into NPs, as drug properties (solubility, metabolism, plasma binding, biodistribution, target tissue accumulation) will no longer be constrained to the same extent by drug chemical composition, but also become in-part the function of the physicochemical properties of the NP. The combination of optimally designed drugs with optimally engineered polymeric NPs opens up the possibility of improved clinical outcomes that may not be achievable with the administration of drugs in their conventional form. In this critical review, we aim to provide insights into the design and development of targeted polymeric NPs and to highlight the challenges associated with the engineering of this novel class of therapeutics, including considerations of NP design optimization, development and biophysicochemical properties. Additionally, we highlight some recent examples from the literature, which demonstrate current trends and novel concepts in both the design and utility of targeted polymeric NPs (444 references).

Temporal control over cellular targeting through hybridization of folate-targeted dendrimers and PEG-PLA nanoparticles

Polymeric nanoparticles (NPs) and dendrimers are two major classes of nanomaterials that have demonstrated great potential for targeted drug delivery. However, their targeting efficacy has not yet met clinical needs, largely because of a lack of control over their targeting kinetics, which often results in rapid clearance and off-target drug delivery. To address this issue, we have designed a novel hybrid NP (nanohybrid) platform that allows targeting kinetics to be effectively controlled through hybridization of targeted dendrimers with polymeric NPs. Folate (FA)-targeted generation 4 poly(amidoamine) dendrimers were encapsulated into poly(ethylene glycol)-b-poly(D,L-lactide) (PEG-PLA) NPs using a double emulsion method, forming nanohybrids with a uniform size (~100 nm in diameter) at high encapsulation efficiencies (69-85%). Targeted dendrimers encapsulated within the NPs selectively interacted with FA receptor (FR)-overexpressing KB cells upon release in a temporally controlled manner. The targeting kinetics of the nanohybrids were modulated using three different molecular weights (MW) of the PLA block (23, 30, and 45 kDa). The release rates of the dendrimers from the nanohybrids were inversely proportional to the MW of the PLA block, which dictated their binding and internalization kinetics with KB cells. Our results provide evidence that selective cellular interactions can be kinetically controlled by the nanohybrid design, which can potentially enhance targeting efficacy of nanocarriers.

Nanoscale drug delivery systems for enhanced drug penetration into solid tumors: current progress and opportunities

Poor penetration of anticancer drags into solid tumors significantly limits their efficacy. This phenomenon has long been observed for small-molecule chemotherapeutics, and it can be even more pronounced for nanoscale therapies. Nanoparticles have enormous potential for the treatment of cancer due to their wide applicability as drug delivery and imaging vehicles and their size-dependent accumulation into solid tumors by the enhanced permeability and retention (EPR) effect. Further, synthetic nanoparticles can be engineered to overcome barriers to drag delivery. Despite their promise for the treatment of cancer, relatively little work has been done to study and improve their ability to diffuse into solid tumors following passive accumulation in the tumor vasculature. In this review, we present the complex issues governing efficient penetration of nanoscale therapies into solid tumors. The current methods available to researchers to study nanoparticle penetration into malignant tumors are described, and the most recent works studying the penetration of nanoscale materials into solid tumors are summarized. We conclude with an overview of the important nanoparticle design parameters governing their tumor penetration, as well as by highlighting critical directions in this field.

Photoswitchable nanoparticles for triggered tissue penetration and drug delivery

We report a novel nanoparticulate drug delivery system that undergoes reversible volume change from 150 to 40 nm upon phototriggering with UV light. The volume change of these monodisperse nanoparticles comprising spiropyran, which undergoes reversible photoisomerization, and PEGylated lipid enables repetitive dosing from a single administration and enhances tissue penetration. The photoswitching allows particles to fluoresce and release drugs inside cells when illuminated with UV light. The mechanism of the light-induced size switching and triggered-release is studied. These particles provide spatiotemporal control of drug release and enhanced tissue penetration, useful properties in many disease states including cancer.

Uptake and toxicity studies of poly-acrylic acid functionalized silicon nanoparticles in cultured mammalian cells

Poly-acrylic acid (PAAc) terminated silicon nanoparticles (SiNPs) have been synthesized and employed as a synchronous fluorescent signal indicator in a series of cultured mammalian cells: HHL5, HepG2 and 3T3-L1. Their biological effects on cell growth and proliferation in both human and mouse cell lines have been studied. There was no evidence of in vitro cytotoxity in the cells exposed to PAAc terminated SiNPS when assessed by cell morphology, cell proliferation and viability, and DNA damage assays. The uptake of the nanocrystals by both HepG2 and 3T3-L1 cells was investigated by confocal microscopy and flow cytometry, which showed a clear time-dependence at higher concentrations. Reconstructed 3-D confocal microscope images exhibited that the PAAc-SiNPs were evenly distributed throughout the cytosol rather than attached to outer membrane. This study provides fundamental evidence for the safe application and further modification of silicon nanoparticles, which could broaden their application as cell markers in living systems and in micelle encapsulated drug delivery systems.

Safety assessment of nanomaterials: implications for nanomedicine

Nanotechnologies offer exciting opportunities for targeted drug delivery which is anticipated to increase the efficacy of the drug and reduce potential side-effects, through the reduction of the dose of the drug in bystander tissues and an increase of the drug at the desired target site. Nevertheless, understanding whether the nano-scale carriers themselves may exert adverse effects is of great importance. The small size may enable nanoparticles to negotiate various biological barriers in the body which could, in turn, give rise to unexpected toxicities. On the other hand, the potential of nanoparticles to cross barriers can also be exploited for drug delivery. Determining the fate of nanoparticles following their therapeutic or diagnostic application is critical: are nanoparticles excreted, or biodegraded, or do they accumulate, potentially leading to harmful long-term effects? The bio-corona of proteins or lipids on the surface of nanoparticles is a key parameter for the understanding of biological interactions of nanoparticles. In the present review, we discuss some of the major challenges related to safety of nanomedicines.

HP55-coated capsule containing PLGA/RS nanoparticles for oral delivery of insulin

In this work, we designed and developed a two-stage delivery system composed of enteric capsule and cationic nanoparticles for oral delivery of insulin. The enteric capsule was coated with pH-sensitive hydroxypropyl methylcellulose phthalate (HP55), which could selectively release insulin from nanoparticles in the intestinal tract, instead of stomach. The biodegradable poly(lactic-co-glycolic acid) (PLGA) was selected as the matrix for loading insulin. Eurdragit(®) RS (RS) was also introduced to the nanoparticles for enhancing the penetration of insulin across the mucosal surface in the intestine. The nanoparticles were prepared with the multiple emulsions solvent evaporation method via ultrasonic emulsification. The optimized nanoparticles have a mean size of 285nm, a positive zeta potential of 42mV. The encapsulation efficiency was up to 73.9%. In vitro results revealed that the initial burst release of insulin from nanoparticles was markedly reduced at pH 1.2, which mimics the stomach environment. In vivo effects of the capsule containing insulin PLGA/RS nanoparticles were also investigated in diabetic rat models. The oral delivered capsules induced a prolonged reduction in blood glucose levels. The pharmacological availability was found to be approximately 9.2%. All the results indicated that the integration of HP55-coated capsule with cationic nanoparticles may be a promising platform for oral delivery of insulin with high bioavailability.

Compact zwitterion-coated iron oxide nanoparticles for biological applications

The potential of superparamagnetic iron oxide nanoparticles (SPIONs) in various biomedical applications, including magnetic resonance imaging (MRI), sensing, and drug delivery, requires that their surface be derivatized to be hydrophilic and biocompatible. We report here the design and synthesis of a compact and water-soluble zwitterionic dopamine sulfonate (ZDS) ligand with strong binding affinity to SPIONs. After ligand exchange, the ZDS-coated SPIONs exhibit small hydrodynamic diameters, and stability with respect to time, pH, and salinity. Furthermore, small ZDS coated SPIONs were found to have a reduced nonspecific affinity (compared to negatively charged SPIONs) toward serum proteins; streptavidin/dye functionalized SPIONs were bioactive and thus specifically targeted biotin receptors.